Plate and Shell type welded plate heat exchangers are previously known, which are composed of a plate pack formed by heat exchange plates and a shell surrounding it, functioning as a pressure vessel. The core of the heat exchanger is usually formed by a plate pack composed of circular heat exchange plates, where the plates have been welded tightly together at openings therein and/or at the perimeters of the plates. A primary circuit of the heat exchanger is formed between the openings in the plates and a secondary circuit between connections of the shell surrounding the plate pack, so that a heat exchange medium of the primary side flows in every other plate space and a heat exchange medium of the secondary side in every other plate space.
The performance of the heat exchanger can be improved by arranging two heat exchange mediums to pass each other several times. A heat exchanger wherein heat exchange mediums pass each other more than once is typically called a multi-pass heat exchanger. The multiple passes can be carried out e.g. by inserting stopper plates on the shell side and/or the pack side of the heat exchanger. These stopper plates direct the medium back and forth across the plate pack several times to achieve the multi-pass effect.
In heat exchangers composed of circular heat exchange plates, in which the plate pack is placed inside a cylindrical shell, it is problematic to arrange multiple passes in the secondary side inside the shell in such a way that there is no by-pass flow between the plate pack and the shell. The by-pass flow reduces efficiency of the plate heat exchanger. Moreover, the additional structures in the heat exchanger structure might weaken the pressure-tightness of the heat exchanger and so the structure of the heat exchanger has commonly been kept as simple as possible.